The goal of the project is to determine whether juxtamedullary nephrons of the kidney exhibit the ability to exert feedback control on their rate of glomerular filtration. In a series of open-feedback-loop experiments, tubuloglomerular feedback will be measured in juxtamedullary and superficial nephrons in hydropenic and diuretic hamster kidneys. The effects of Henle's loop perfusion rate and perfusate composition on filtration rate in the same nephron will be measured. In a series of closed-feedback-loop experiments, it will be determined whether tubuloglomerular feedback mediates juxtamedullary nephron autoregulation of glomerular filtration rate. By measuring juxtamedullary nephron filtration rate without interrupting the mechanism of tubuloglomerular feedback during diuresis, it will be determined if the increased loop of Henle flow due to the diuresis leads to a reduction in single nephron filtration rate by activating juxtamedullary nephron tubuloglomerular feedback. An integral part of the research is a mathematical model of Henle's loop function. The model, which stimulates loop water and solute reabsorption, is used for experimental design and data interpretation.